Method of protecting metal surfaces during heat treatment



United States Patent 3,415,691 METHOD OF PROTECTING METAL SURFACESDURING HEAT TREATMENT Ramesh Vyas, Birmingham, England, assignor toFoseco International Limited, Birmingham, England No Drawing. Filed Nov.3, 1964, Ser. No. 408,672 Claims priority, application Great Britain,Nov. 7, 1963, 44,025/63 15 Claims. (Cl. 148--6.11)

ABSTRACT OF THE DISCLOSURE A method of treating metal surfaces toprotect the surfaces against scale formation during heat treatment suchas annealing including applying to the surface a composition of avitreous material such as powdered glass, a refactory filler such assilica and ferrosilicon.

This invention relates to the treatment of metals and more particularlyto means for preventing or reducing oxidation of the surface of metalobjects such as billets, slabs and castings during heat treatmentoperations applied thereto.

It is commonplace in the metal-working industry that metals aresubjected to various heating operations, e.g., for annealing and/orstress relieving or in conjunction with hot working-operations. Duringsuch heating the surfaces of the metal are especially liable to sufferthe effects of oxidation phenomena, e.g., the formation of a layer ofthe metal oxide on the surface of the metal object and/or the selectiveoxidation of an alloying constituent. The latter phenomenon isparticularly exemplified by the surface decarburisation of steels orother ferrous alloys. The effects may be exhibited by the formation ofan oxide scale or, in the case of decarburisation by the creation of asurface layer which has changed chemical and physical characteristicsand which subsequently must, in consequence, be machined away. Metallosses incurred in these ways can be severe and, therefore, very costly.Thus, for example, a steel billet or slab, prior to rolling into rail orsheet, is commonly placed in a socalled heat-treating furnace in whichit is uniformly heated. The furnace must of course be opened to removethe billet or slab. Oxidation and possibly decarburisation there takesplace and the loss of metal from such causes may be as high as 3% byweight of the metal. It is therefore, of prime importance that somemeans should be found of shielding the metal surfaces from an oxidisingatmosphere during any heat-treating operation.

It is well known that, in order to reduce these oxidation losses, smallcastings may be subjected to their necessary heat treatment in acontrolled inert or non-oxidising atmosphere. Although this expedienthas been fairly successful in many instances and, indeed, is widelypracticed, there are certain serious objections to its generalapplication. An obvious non-oxidising atmosphere for use in anatmosphere of nitrogen but this may give undesirable side effects, e.g.,nitriding of steel when heated in nitrogen atmosphere. Argon may be usedinstead of nitrogen but is of course very expensive. In any event,expensive equipment is necessary, e.g., gas flow control apparatus andspecially constructed furnaces in which air may be prevented fromentering by induction. Furnaces in which the atmosphere may becontrolled are suitable for heating small objects such as tools but itis impractical to make use of such apparatus to heat large objects suchas billets and slabs which may weigh several tons. Moreover, if themetal object is required to be removed from the furnace whilst still ina heated condition (e.g., a steel billet or Patented Dec. 10, 1968 slabfor rolling), fairly severe oxidation can still take place on exposureto the atmosphere.

Alternative means of protecting the surfaces of metal articles to beheated have therefor been developed. Thus, it is known to apply a paintto the surfaces of the metal before subjecting it to the heat treatment.Such paints may include materials which are preferentially oxidised(e.g., powdered aluminium, ferrosilicon or silicon and are thus intendedto function in a sacrificial manner. Other types of paint depend uponthe physical exclusion of the atmosphere from the surface by the abilityof their constituents to form a glaze when heated. Materials used insuch paints are mixtures of various refractory oxides, slags, silica andground glass. The preferentially oxidised materials mentioned above havealso been included in the glaze-forming perparations. Varying degrees ofsuccess may be achieved by the use of these known paints which aregenerally applied to the metal surface as a layer of the order of A2.However, none of them has been found to be capable of consistentlyreducing oxidation losses to a satisfactory degree although, in someinstances, the amount of metal lost as oxide scale has been reduced byas much as 70%. A reduction in losses even of this magnitude is howeverconsidered unsatisfactory in that the degree of oxidation still sufferedis inevitably accompanied, in the case of steels, by surfacedecarburisation, thus necessitating a machining operation to remove thedecarburised layer.

It may be observed in this connection that where a mechanical protectionis to be achieved, as by a glaze, it is important that there should beno cracks or pinholes in the protective layer for if there are theoxidation and decarburisation eifect tends to spread beneath the glazefar beyond the crack or pinhole itself.

It is an object of the present invention to provide a method ofprotecting metal surfaces which comprises applying a composition theretowhich, when applied to metal articles to be treated, affords asubstantially improved measure of protection against surface oxidationand surface decarburisation during heat treatment, so that the extent ofresidual metal oxide formation is at most 20% by weight of that whichoccurs when no preventive measures are taken, i.e., at least four fifthsof the normal loss is avoided. The residual oxidation is then at so lowa level that it may be wholly unnecessary to carry out any machining ofthe surface for removal of oxidised or decarburised strata.

It has been found, as a result of considerable research and experimentthat a protective layer, in order to be capable of achieving this degreeof success, must be able to form a continuous film on the surface of themetal article which is imperivious to gases, and must adhere firmly tothe metal surface. It has also been discovered that the coatingcomposition must be formulated such that the applied coating does notcompletely fuse to form a glaze since, in such acase, the film acts asan oxide ion carrier and scaling can become very severe indeed; worse,in fact than on a similar, untreated specimen. Moreover, as a practicalmatter the composition used to apply the layer should have such fluiditythat it may be applied by spraying, dipping, brushing or any otherconventional method.

It is an object of the present invention to provide an improved methodof treating metal slabs and billets to minimise or inhibit surfaceoxidation which makes use of novel compositions for application to themetal surfaces.

Accordingly, there is provided a method for the treatment of slabs,billets and other articles of metal, to reduce or inhibit surfaceoxidation during heat treatment thereof, which comprises applying to thesurface thereof a layer of a composition containing ferrosilicon, finelydivided refractory material and a fusible glaze-forming material.

It is generally, most convenient to apply the aforesaid compositions inthe form of dispersions, suspensions or slurries and for that reason itis desirable to include with the composition as applied, in addition toa liquid carrier for the solid ingredients, a dispersing agent (i.e., anagent which will aid in holding the particulate ingredients insuspension in the carrier liquid). It is also frequently desirable toinclude an adhesive substance which will aid in anchoring the appliedlayer to the metal surface.

In a particular form of the invention the composition further includesingredients which when the coated metal is heated react with one anotherexothermically. It is found that in this way the fusing of theglaze-forming material is facilitated and improved results may beobtained.

Referring to these various ingredients:

The ingredient ferrosilicon is well known and requires no furthercomment.

The finely divided refractory material may be, for example, alumina,silica, magnesia or any other refractory oxide or a mixture of any ofthese, or any other refractory material such as a refractory silicate.

The fusible glaze-forming material may be powdered glass or any materialknown per se for use as a ceramic glazing material. These are usuallymixtures of silicates, borates or phosphates with metal oxides, e.g.,iron oxide, or lead oxide. Slagging agents known for use in the iron andsteel industry may also be employed.

When, as is preferred, the composition is in the form of a suspension,dispersion or slurry in a liquid carrier, any liquid carrier can beused. Water is generally preferred from the standpoint of convenienceand cheapness. Any other volatile or inflammable liquid can be employed,e.g., an alcohol, but the latter are generally less preferable asintroducing fire and explosion hazards.

The dispersing agent may be any such compound known per se, e.g., amontmorillonite gel. Adhesives present in the composition may be gums orresins, r bentonite clay or mixture of these.

Where it is desired that the composition should contain exothermicallyreacting ingredients, these will usually consist of an oxidisablesubstance and an oxidising agent. The former may be the ferrosiliconwhich is in any event present in the composition but there mayadditionally be present a proportion of finely divided aluminium. Theoxidising agent may be most conveniently an alkali metal or alkalineearth metal nitrate or chlorate, iron oxide (Fe O or Fe O manganesedioxide and mixtures of any of these.

Where aluminium and an oxidising agent is present it is usuallypreferable also to include a proportion of a fluoride, e.g., alkalimetal or alkaline earth metal fluoride, aluminium fluoride or a mixedfluoride such as sodium aluminium fluoride or potassium aluminiumfluoride or a complex fluoride such as a silica fluoride, boro fluorideor titano fluoride. However it is also suitable to use the fluoride inthe absence of the aluminium and/or the oxidising agent.

The following formulations are illustrative of protective compositionsuseful in carrying out the novel method of this invention:

Example 1 Parts by weight Refractory (e.g. alumina or silica) (l00 meshB.S.S.) Ground glass (-200 mesh B.S.S.) 3 Ferrosilicon (200 mesh B.S.S.)34 Aluminium powder (100 mesh B.S.S.) 14 Fluoride (60 mesh B.S.S.) 4Oxidising agent (60 mesh B.S.S.) 15 Bentonite 7 Rosin pitch 3 Alkylammonium montmorillonite gel The whole of the foregoing ingredients aremixed to a paste and diluted with isopropyl alcohol in the proportions100 gm. paste: 100 cc. isopropyl alcohol.

The gel referred to is prepared by thorough kneading together 8.7 partsby weight of alkyl ammonium montmorillonite and 4.3 parts by weight ofmethyl alcohol. The dough thus formed is added to 87 parts by weighttoluene whilst stirring gently. Finally the whole mixture is stirredvigorously for about three minutes, after which it is transferred to anair tight container and allowed to stand for 24 hours before use.

Example 2 Parts by weight Refractory material (eg. alumina or silica)(-l00 mesh B.S.S.) 3O Powdered glass (-200 mesh B.S.S.) 2 /2Ferrosilicon (200 mesh B.S.S.) 6 Sodium fluoride (-l0() mesh B.S.S.) 2/2 Bentonite 2 Vinsol resin 2 /2 Alkyl ammonium montmorillonite gel 11The gel is prepared in the same way as in Example 1. The 11 parts of theprepared gel were mixed with 66 parts by weight of isopropyl alcoholuntil all lumps were removed and the remaining dry ingredients wereadded in the proportions stated.

Example 3 Parts by weight Refractory material (eg. alumina or silica)(-lO0 mesh B.S.S.) 18 Powdered glass (60 mesh B.S.S.) 2 Ferrosilicon(200 mesh B.S.S.) 28 Powdered aluminium (l00 mesh B.S.S.) l2 Fluoride100 mesh B.S.S.) 5 Oxidising agent (nitrate, iron oxide or the like (60mesh B.S.S.) l2 Bentonite 7 Water soluble resin binder (e.g. ureaformaldehyde, phenol formaldehyde resin) Aqueous suspension agent (e.g.a gum) 4 Sodium silicate solution (4.8% solids) 4 Buffer (eg. boric acidor mono sodium orthophosphate) All the ingredients except sodiumsilicate, resin and the suspension agent are mixed together in theproportions stated (88 parts by weight) and added to parts weight ofwater. The remaining ingredients are then stirred into the mixture.

Example 4 Parts by weight parts of above mixture are mixed with 25 partsof isopropyl alcohol exactly as in Example 1.

The compositions thus produced are in the form of slurries which may beapplied, preferably by spraying, to the surfaces of the metal to besubjected to heat treatment. Adherence of the resulting layers to metalsurfaces is very good, cohesion being given to the applied coating bythe resin and bentonite content. The gel is included in the compositionas a suspension agent and, even though some settling of the solidconstituents may occur, redispersion is effected very readily.

The anti-oxidising efliciency of the foregoing compositions is clearlydemonstrated by the results of the following tests. Steel specimens ofcylindrical shape, 1" diameter and weighing approx. 100 gm. were heatedfor 1 hour in a furnace at 1100 C. This resulted in surface oxidation ofthe specimens to the extent that of the metal was lost by scaling. Itwas found that these losses were consistently reduced by at least 85%when identically similar specimens were subjected to the same heatingafter spraying the surfaces with any of the compositions shown in theforegoing Examples 1, 2 and 3. (Naturally, appropriate precautions mustbe taken with the first two compositions, which contain isopropylalcohol.) Similar steel specimens, when coated with the compositionaccording to Example 4, were able to Withstand a heat treatment at 1300C. for a similar period without undergoing a more severe oxidation. Itis remarkable that such a degree of protection is afforded by as littleas a 0.025" thickness layer of these novel compositions. In fact,increasing the thickness of the layer above 0.025 confers very littleextra protection against surface oxidation.

It should be pointed out that, in the case of formulations utilising anaqueous medium, the self life of the composition is liable to be shortbecause of chemical reaction between constituents. Such reactions may beavoided or minimised by the addition of further ingredients to controlthe pH of the aqeous phase. For example, Example 3 employs boric acidand monosodium orthophosphate as buffering agents. Corrosion of suchingredients as ferrosilicon may be further inhibited by precoating them,e.g., with insoluble resins.

The novel method of this invention is suitable, therefore, forprotecting metal billets and slabs which are to be heated before hotworking operations. The method is also useful for protecting the surfaceof finished metal articles which are to undergo heat treatment, e.g.,tools which have been formed and machined to size but which must then beannealed, tempered or otherwise heat treated.

I claim as my invention:

1. In a method of protecting metal surfaces against scale formationduring heat treatment the step of applying to such surfaces acomposition cousitsing essentially of vitreous material, a refractoryfiller selected from the group consisting of aluminia, silica, magnesia,refractory silicates and mixtures thereof, and ferrosilicon, inproportions, per one part by weight vitreous material, of from about 5to about 30 parts by weight refractory filler, and from about 1 to aboutparts by weight ferrosilicon.

2. The method of claim 1 wherein the vitreous material of thecomposition is a glass.

3. The method of claim 1 wherein the refractory filler of thecomposition is silica.

4. The method of claim 1 wherein the composition contains up to about10% by weight of an oxidising agent selected from the group consistingof alkali metal nitrates, alkaline earth metal nitrates, alkali metalchlorates, alkaline earth metal chlorates, iron oxide, manganese Oxideand mixtures thereof.

5. The method of claim 4 wherein the composition contains up to about10% by weight of a fluoride.

6. The method of claim 5 wherein the composition contains up to about10% by weight a fluoride.

7. The method of claim 1 wherein the composition contains up to about10% by weight finely divided aluminium.

8. The method of claim 1 wherein the composition is in the form of asuspension of the ingredients in a liquid carrier.

9. The method of claim, 8 wherein the liquid carrier is water.

10. The method of claim 1 wherein the composition contains up to about10% by weight of a suspending agent 11. The method of claim 9 whereinthe composition contains up to about 10% by weight of an adhesivesubstance.

12. The method of claim 8 wherein the composition contains up to about10% by weight of an oxidising agent selected from the group consistingof alkali metal nitrates, alkaline earth metal nitrates, alkali metalchlorates, alkaline earth metal chlorates, iron oxide, manganese oxideand mixture thereof.

13. The method of claim 8 wherein the composition contains up to about10% by weight finely divided aluminium.

14. The method of claim 1 wherein the refractory filler of thecomposition is alumina.

15. The method of claim 1 wherein the refractory filler of thecomposition is magnesia.

References Cited UNITED STATES PATENTS 1,359,281 11/1920 Scott 148-142,686,134 8/1954 Wooding et al. 148-24 X 2,806,801 9/1957 Leston 14826 X2,865,798 12/1958 Hindson 14826 2,898,253 8/1959 Schneider et al. N148l3.1 X 2,951,000 8/ 1960 Kennedy et al 148-26 3,037,878 6/1962 Cowleset a1 10648 X 3,167,450 1/1965 Koibuchi et al. 117206 3,178,322 4/1965Schneider 148-13.1 X 3,196,537 7/1965 Groman et al. 1l7206 X 3,253,9505/1966 Wasserman et al. 117206 RALPH S. KENDALL, Primary Examiner.

US. Cl. X.R.

